For a great many years the diaphragm-type cell has been used commercially to electrolyze brine into chlorine and caustic. Almost from the beginning, asbestos fibers have been highly regarded as suitable raw material for the preparation of diaphragm separators for such cells. Most such diaphragms have been formed as a matted fibrous coating on foraminous cathodes, e.g by suction induced deposition of solid matter from a slurry of the asbestos fibers. These supported asbestos diaphragms proved to be quite serviceable for duty as the hydraulically permeable separators in percolating electrolytic cells and were, therefore, widely adopted in the chlor-alkali industry.
In recent years however, the industry has been faced with a rapid escalation in the cost of electric power and other operating expenses. Accordingly, more and more effort has been focused on increasing the operating efficiency of brine electrolysis with considerable emphasis on developing improved electrode assemblies which are more resistant to dimensional changes and deterioration under the continuous, heavy duty service conditions of the modern chlor-alkali cell.
Most of the suggestions for improving the stability and service life of asbestos diaphragms which are found in the published prior art involve the incorporation of some sort of binder material, usually a synthetic organic, polymeric resin. A wide variety of such polymeric resins have been proposed as well as many different techniques for incorporating them in the asbestos diaphragms. In order to illustrate the present state of this art, attention is now directed to the following representative references:
British Pat. No. 1,410,313 (Fenn et al) assigned to Diamond Shamrock Corporation (which is the British counterpart of U.S. Pat. No. 4,410,411): This reference teaches that excellent, dimensionally stable, cathode supported diaphragms can be made using only the simultaneous co-deposition technique from a single composite slurry of asbestos fibers and particulate, thermoplastic fluorocarbon polymer binder.
U.S. Pat. No. 3,853,720--Korach et al: This patent teaches that an asbestos diaphragm can be made more durable without losing electrolyte permeability by impregnating same with a minor amount of a hydrophilic fluorocarbon resin in solution in a organic solvent.
British Pat. No. 1,533,429--BASF Wyandotte Corporation: This patent recommends using hydrophobic fluorocarbon resins as the binding or cementing agent and indicates that such resinous additives can be incorporated not only by using solvent impregnation techniques but also by incorporating the resin directly into the aqueous slurry of asbestos fibers from which the diaphragm is to be formed.
U.S. Pat. No. 4,065,534--Rechlicz et al: This reference states that, for best results when adding the hydrophobic resin binder directly to the aqueous slurry of asbestos fibers, the aqueous medium used to prepare said slurry should be a substantially salt-free solution of alkali metal hydroxide.
British Pat. No. 1,498,733--Hooker Chemicals and Plastics Corporation: This patent indicates that a more uniformly resin-reinforced asbestos diaphragm can be produced by subjecting a preformed and dried asbestos mat to subsequent impregnation with a separate dilute slurry of thermoplastic resin powder rather than by co-depositing both asbestos and resin from a single slurry of the two raw materials.
Moreover, there are disadvantages of conventional asbestos diaphragm modifiers which are overcome by the diaphragm according to the present invention. As far as Applicants are aware, when a conventional diaphragm is operated without spacers between the anode and diaphragm (zero gap), there is an initial voltage improvement, but the voltage improvement does not stay. Instead the voltage increases. By the end of the first 2-months of operation, the voltage will be higher than from a conventional cell with spacers. Moreover, when the conventional diaphragm is removed from zero gap service, it falls apart. The surface is swollen and the fibers are penetrating the anode.
Through approaches such as those described in the above references, important gains in the service life and stability of asbestos-type diaphragm separators have already been achieved. However, these gains have not been made without considerable cost because of the expensive nature of the perfluorocarbon polymers which are preferred for use as the resin binders as well as the extra steps involved in the fabrication of the reinforced diaphragms. Accordingly, much research continues to be directed towards finding alternative methods of increasing diaphragm life and operating stability at lower costs.